The American Hill district is in southwestern Sierra Counry about five miles east of the town of Alleghany. It is both a lode and placer district, but the placer deposits have been more important. It includes the Cornish House area.
The district is chiefly underlain by slate. Serpentine and amphibolite are to the west, and granodiorite is just to the east. Extensive gravel deposits are part of a tributary to the Forest channel of the Tertiary Yuba River, which extends in a southwest direction through the district. The northern part of the district is covered by andesite. Dioritic dikes often are associated with the gold-quartz veins.
The gravels are quartzitic and contain coarse gold. One of the gravel deposits is as much as 300 feet thick and covered with clay and sand. The quartz veins are lenticular and occur either in the slate or near the slate-granodiorite contact. The Ore bodies contain free gold with pyrite, arsenopyrite, and galena. Sometimes carbon is found in cavities in the quartz. Some high-grade pockets have been fonnd, but most of ore averages less than .5 ounces per ton.
Placer: American Hill, Bear Creek, Excelsior, Mable Mertz, Yellow Jacket. Lode: Comet, Ironsides, Jim Crow, Lonesome Pine, Pilgrim, Von Humboldt.
Lindgren, Waldemar, 1900, Colfax folio: U. S. Geol. Survey Geol. Atlas of the U.S., folio 66, 10 pp.
Lindgren, Waldemar, 1911, Tertiary gravels of the Sierra Nevada: U.S. Geol Survey Prof. Paper 73, pp. 142-143.
MacBoyle, Errol, 1920, Sierra County, American Hill district: Calif. Min. Bur. Rept. 16, pp. 5-6.